The invention relates generally to semiconductor devices and integrated circuit fabrication and, in particular, to device structures and fabrication methods for the bipolar junction transistors of a bipolar device.
Bipolar junction transistors may be found, among other end uses, in high-frequency and high-power applications. In particular, bipolar junction transistors may be used in microwave power amplifiers used in wireless communications systems and power amplifiers in mobile devices. Bipolar junction transistors may also be used in high-speed logic circuits.
Bipolar junction transistors are three-terminal electronic devices that include semiconductor regions defining an emitter, an intrinsic base, and a collector. An NPN bipolar junction transistor includes two regions of n-type semiconductor material constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base situated between the two regions of n-type semiconductor material. A PNP bipolar junction transistor includes two regions of p-type semiconductor material constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base situated between the two regions of p-type semiconductor material. The emitter, intrinsic base, and collector define a pair of p-n junctions, namely a collector-base junction and an emitter-base junction, across which the conductivity type of the respective semiconductor materials changes. A voltage applied across the emitter-base junction controls the movement of charge carriers that produces charge flow between the collector and emitter.
Improved device structures and fabrication methods are needed for a bipolar device.